and increasing proportion of soldiers were now of native, rather foreign origin, with decreasing participation of szlachta, who however monopolized the command and dominated the officer corps. The highest ranks belonged to wealthy magnates, with the notable exception of Stefan Czarniecki, a nobleman of more humble origins. Czarniecki introduced guerrilla warfare with heavy participation of peasant masses. Afterwards the last great Polish battlefield successes were made possible by the universal military talent of Jan Sobieski. The one major weakness and backwardness of the Commonwealth defenses was the lack of significant system of modern fortifications throughout most of its territory, which allowed the invading armies to inflict damage disproportionate to military effort. The exceptions were the mouth of the Vistula region and the south-east, where there were powerful, even if neglected fortresses, such as the Kamieniec Podolski (Kamianets-Podilskyi) Castle. The fiscal difficulties affected also the Commonwealth diplomacy and foreign policy. John III kept resident diplomats in several major capitals, but the Sejm imposed limits on what they could undertake. The Sejm also banned long-term resident foreign representations in Warsaw, a prohibition ignored by the papacy and numerous governments. The Sejm itself had rather frequently sent its own missions abroad. The magnates, including top state officials, especially the hetmans, conducted their own foreign activities and at times served foreign powers for their private gain. When French dominated European diplomacy, in Poland the official language was still Latin. The 1684 treaty of Holy League, which ignored the recent Polish military contributions and Polish interests alike, was just one example of the Commonwealth's diplomacy lack of effectiveness. Swedish invasion, empowerment of East Prussia The Swedish invasion of the Commonwealth, known as the Deluge, took place within the context of the Second Northern War. In 1655 the Commonwealth's survival had become endangered, when the huge federation, already critically weakened by the offensive of the Russians, who occupied most of the Grand Duchy of Lithuania, and by Khmelnytsky's Cossacks holding fast to the lands they overran in Ukraine, was subjected to a massive attack by Sweden. The Swedish leaders, emboldened by the Peace of Westphalia gains, including the western Duchy of Pomerania, intended precisely to take advantage of Poland's grave difficulties, hoping to easily take over at least Courland and Prussia, and thus to enforce their total domination over the Baltic Sea area. Worried about the Russian strength and advances in the East Baltic region and